Re: [PATCH bpf-next 1/2] bpf: add bpf_init_inode_xattr kfunc for atomic inode labeling

[Matt Bobrowski <mattbobrowski@google.com>]

On Mon, Apr 27, 2026 at 05:32:47AM +0200, Kumar Kartikeya Dwivedi wrote:
> On Mon, 27 Apr 2026 at 05:24, David Windsor <dwindsor@gmail.com> wrote:
> >
> > On Sun, Apr 26, 2026 at 10:57 PM Kumar Kartikeya Dwivedi
> > <memxor@gmail.com> wrote:
> > >
> > > On Mon, 27 Apr 2026 at 02:16, David Windsor <dwindsor@gmail.com> wrote:
> > > >
> > > > Add bpf_init_inode_xattr() kfunc for BPF LSM programs to atomically set
> > > > xattrs via inode_init_security hook using lsm_get_xattr_slot().
> > > >
> > > > lsm_get_xattr_slot() claims a slot by writing to xattr_count, which BPF
> > > > programs cannot do: hook arguments are not directly writable from BPF.
> > > > To hide this, the BPF-facing API is just bpf_init_inode_xattr(name,
> > > > value), and the verifier transparently rewrites each call into
> > > > bpf_init_inode_xattr_impl(xattrs, xattr_count, name, value). xattrs and
> > > > xattr_count are extracted from the hook context, which the verifier
> > > > spills to the stack at program entry since R1 is clobbered during normal
> > > > execution.
> > > >
> > > > A previous attempt [1] required a kmalloc string output protocol for
> > > > the xattr name. Since commit 6bcdfd2cac55 ("security: Allow all LSMs to
> > > > provide xattrs for inode_init_security hook") [2], the xattr name is no
> > > > longer allocated; it is a static constant. We take advantage of this by
> > > > passing the name directly. Because we rely on the hook-specific ctx
> > > > layout, the kfunc is restricted to lsm/inode_init_security.
> > > >
> > > > Link: https://kernsec.org/pipermail/linux-security-module-archive/2022-October/034878.html [1]
> > > > Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6bcdfd2cac55 [2]
> > > > Suggested-by: Song Liu <song@kernel.org>
> > > > Signed-off-by: David Windsor <dwindsor@gmail.com>
> > > > ---
> > >
> > > The explanation and code make no sense to me. Why not pass xattrs and
> > > xattr_count directly as arguments, even if you choose to restrict the
> > > kfunc to a specific hook? Why does the verifier core need the hack to
> > > spill the context and extract the two arguments?
> > >
> >
> > xattr_count is an output parameter; we cannot currently write to it in
> > bpf as there is no verifier support for writing to int *.  xattrs and
> > xattr_count can be fixed up by the verifier, so we only require the
> > user to pass the name and value.
> 
> Sure, but the kfunc can. Did you try passing them in directly?
> If that doesn't work for some reason, we should fix it instead.

Hm, perhaps this fixup approach might be the simplest in order to
assure the needed safety? Allowing this new BPF kfunc to take xattr
and xattr_count arguments directly from the BPF LSM program could
possibly result in lsm_get_xattr_slot() to go off the rails, no?
Unless, what you're proposing here is to also add some provenance-like
tracking such that the BPF verifier assures that only the BPF LSM
program context arguments end up being supplied to it (if this is
something that doesn't already exist)?.